Update USE_GITHUB so it does not require GH_COMMIT.
Using this new scheme allows only setting the _tag_ or _commit hash_ in
GH_TAGNAME and not having to know the hash for a tag. This scheme will
download a tarball that has a different checksum than before due to a changed
directory name for extraction.
The following MASTER_SITES are provided to retain the old checksum and
directory structure (that require GH_COMMIT):
GH -> GHL GITHUB -> GITHUB_LEGACY
Differential Revision: https://reviews.freebsd.org/D748
Submitted by: amdmi3
Reviewed by: mat, swills, antoine, bdrewery
With hat: portmgr